1. Field of the Invention
The present invention generally relates to a method and device for the rapid drying of coated materials by the application of microwave energy. The invention may be used for the removal of water or organic solvents from coated materials, especially, but not limited to, continuous webs.
2. Brief Description of the Related Art
A variety of industrial products are manufactured in the form of long thin webs which are coated or printed. Examples of these products include wall coverings (e.g., wallpaper), plastic and paper packaging, published materials, textiles, photographic films, plastic transparencies, magnetic media and adhesive tapes. Typically, the coating of these products is performed with the use of a volatile organic compound (VOC) or water. Examples of VOCs that may be used in these processes include methyl ethyl ketone, acetone, toluene, alcohols, and chlorinated solvents. After the web material has been processed, the solvent used is typically removed, thus leaving the desired coating or printing on the web. The removal of these solvents from web materials is typically accomplished through a heating process.
All manufacturers of printed and coated web products are strongly affected by new provisions of the Clean Air Act, which mandate strict controls on the emission of VOCs to the atmosphere. The costs for a VOC emission control system tend to be strongly dependent on the degree of dilution of VOCs in an air stream. Since coated web material is conventionally dried by exposure to hot air streams, air dilution of the VOCs is normally inherent in the drying process. This dilution tends to create large volumes of air which typically need to undergo treatment before the air is released into the atmosphere.
Contaminated air streams are typically treated by either incineration or passage of the air through an adsorbent material. In a typical incineration procedure, the stream is heated to about 600.degree. C. to decompose the VOCs. If the concentration of organics is too dilute, natural gas is typically added such that sufficient combustion may be achieved. It is therefore desirable that the stream of organic contaminants be concentrated before incineration, to lower the amount of additional fuel needed to effect destruction of organics. The use of an air stream with a higher concentration of VOCs requires less additional fuel and, therefore, less overall cost.
Alternatively, adsorbent materials may be used to remove the VOCs from the air. The contaminated air stream may be transferred to an adsorbent column. As the contaminated stream is passed through the column, the VOCs are removed. After the process is completed or when the adsorbent materials become saturated, the VOCs are typically removed from the adsorbent. The purification of the adsorbent is typically accomplished by heating the adsorbent materials to remove the VOCs from the adsorbent material. The removal of the VOCs from the adsorbent tends to be performed such that the VOCs are removed to form an air or inert gas stream containing a relatively high concentration of VOCs. An air or inert gas stream containing a relatively high concentration of VOCs is typically more economical to treat.
An alternate method of treatment of contaminated streams is by recovery of the VOCs from the air stream. To recover the VOCs, the air stream is typically passed through a cooling system which allows the VOCs to condense out of the air stream. The efficiency of recovering solvents in this manner tends to be dependent on the concentration of the VOCs within the air stream. To achieve an economically viable recovery system the VOCs typically need to be relatively concentrated.
In general, water-based coatings, while desirable due to the low toxicity of the solvent, are much harder to evaporate than volatile organic materials. A typical hot air drying system may require seconds to minutes to dry a coated web which has been treated with water. This may require relatively large heating systems which generate large amounts of relatively dilute contaminated air streams. It would be desirable to create a system which would allow a more rapid drying of water-coated web materials, thus creating a more concentrated contaminated air stream.
It is therefore desirable to create a system by which solvents, such as water or VOCs, may be evaporated from coated materials such that the solvents are carried from the materials in an air stream containing a relatively high concentration of the solvent. Additionally, it is further desirable that the drying be accomplished in a relatively short time span. By rapidly drying coated materials to form an air stream containing a relatively high concentration of solvent, both heating costs and waste treatment costs may be reduced.